The invention relates to a method for producing smooth metal oxide layers, especially indium tin oxide layers, on substrates that are used in particular for the production of organic light-emitting diodes. The invention relates further to a substrate coating of indium tin oxide.
Organic light-emitting diodes (OLEDs) are generally produced by applying a transparent conducting layer onto a glass substrate and then structuring the conducting layer so that its acts as an electrode. Various layers of organic materials are then deposited on this transparent electrode. These organic layers are very thin, generally on an order of magnitude of 10 nm. To prevent short circuits or other defects that can be caused by spikes or edges on the surface, the transparent electrode must have a very smooth surface. The display is ultimately completed by a counter-electrode, which is generally metallic, and is then encapsulated.
Smooth indium tin oxide layers (referred to in the following as ITO layers) are often deposited by way of an ion-assisted sputtering, or ion plating, process with which smooth layers can be obtained at low temperatures. Among these processes the Skion process, for example, is familiar. However, in addition to a sputter source that acts as a cathode, ion-assisted sputtering needs an ion source; this means a considerable increase in plant costs, which is automatically reflected in the production costs for the manufacture of OLED displays of this kind. The disadvantage of using customary, standard sputtering processes is that they require an additional, mechanical polishing step and thus an additional process step. This likewise increases the production costs for OLED displays of this kind.
Where customary standard sputter sources are used, especially the DC magnetron process, the layers are applied onto substrates at temperatures above the material's recrystallization temperature (approx. 150° C. for ITO) in order to obtain good electrical and optical properties. Usually, the substrate is heated to temperatures of about 200° C. This heating process is carried out because at lower temperatures, the resistivities and transmittances of the indium tin oxide layers do not satisfy the electrical and optical requirements. With such methods, film growth may be microcrystalline, and spikes can form on the film surface. Experiments have shown that ITO layers deposited at a substrate temperature of 200° C. by DC magnetron sputtering have an rms roughness of 2.3 nm and a maximum roughness of 16.1 nm. If one applies the thin organic-material layers required for the production of organic light-emitting diodes onto a layer like this, a mechanical polishing step is necessary in order to avoid short circuits or other defects caused by spikes or edges on the surface. However, this makes the production of organic light-emitting diodes of this kind more complicated and also increases the production costs.